TALAT Lecture 4500
Resistance Welding
23 pages, 25 figures
Basic Level
prepared by Lutz Dorn, Technische Universität, Berlin
Objectives:
−
−
−
−
−
−
to describe the spot welding characteristics of aluminium and its alloys,
the spot welding process,
the choice of process parameters,
strength values,
electrode life and
requirements for quality assurance
Prerequisites:
- general engineering background,
- metallurgy and physical properties of aluminium and
- surface characteristics (e.g. TALAT lecture 5101)
Date of Issue: 1994
 EAA - European Aluminium Association
4500 Resistance Welding
Table of Contents
4500 Resistance Welding .........................................................................................2
4500.01 Introduction to Spot Welding ................................................................. 3
Suitability of Aluminium and its Alloys for Spot Welding .....................................3
Comparison of Physical Properties of Aluminium and Unalloyed Steel.................4
Resistances during Spot Welding of Steel and Aluminium.....................................5
Constitution of the Oxide Film ................................................................................6
Surface Pretreatment ................................................................................................7
Contact Resistance after Surface Pretreatment ........................................................8
Effect of Storage Time on the Contact Resistance ..................................................9
Peltier Effect (Schematic) ........................................................................................9
Characteristics of Differently Designed Spot Welding Machines .........................10
Relative Voltage Drop of a Resistance Welding Machine ....................................11
Factors Influencing the Life of Electrodes.............................................................12
Influence of Storage Time on Life of Electrodes...................................................12
Influence of Machine Design and Current Type on Life of Electrodes .................13
4500.02 The Process of Resistance Spot Welding .............................................. 14
Current-Force Diagram for Spot Welding Aluminium..........................................14
Recommended Values for Spot Welding with Direct Current ..............................15
Example of a Field of Suitable Welding Parameters .............................................16
Minimum Values for Spot Welding Aluminium Parts ..........................................16
Effect of Weld Spot Diameter on the Shear Strength ............................................17
Correlations between Surface Pretreatment, Weld Strength and Electrode Life ...18
Repeated Tensile Stress Fatigue Strength of One-Spot Samples...........................19
Fatigue Tests Using Specimens Similar to Engineering Parts ...............................19
Fatigue Strength of Spot Welded 1.6 mm Thick 2024 cl Sheets ...........................20
4500.03 Quality Assurance..................................................................................... 21
Allowable Imperfections of Spot Welds in Aluminium Alloys (According to
Aeronautical Standards) Determined by Metallography........................................21
Allowable Imperfections of Spot Welds in Aluminium Alloys (According to
Aeronautical Standards) Determined by X-Ray Examination ...............................21
4500.04 Literature................................................................................................... 22
4500.05 List of Figures............................................................................................ 23
TALAT 4500
2
4500.01
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
♦
Introduction to Spot Welding
Suitability of aluminium and its alloys for spot welding
Comparison of physical properties of aluminium and unalloyed steel
Resistances during spot welding of steel and aluminium
Constitution of the oxide film
Surface pretreatment
Contact resistance after surface pretreatment
Effect of storage time on the contact resistance
Peltier effect (schematic)
Characteristics of differently designed spot welding machines
Relative voltage drop of a resistance welding machine
Factors influencing the life of electrodes
Influence of storage time on life of electrodes
Influence of machine design and current type on life of electrodes
Suitability of Aluminium and its Alloys for Spot Welding
The suitability of aluminium and its alloys for spot welding depends mainly on 3
factors:
−
−
−
surface condition
chemical composition
metallurgical condition.
Because of its high affinity to oxygen, the free surface of aluminium is covered by a
dense, tightly adhering oxide film. This film is a non-conductor of electricity and must,
therefore, be removed prior to welding. This is generally accomplished mechanically by
brushing or chemically by etching. The oxide film, if it comes in direct contact with the
contacting electrode, would lead to a local accumulation of heat at the contacting region
and metal pick-up on electrode, i.e., the electrode life would be very limited.
Both electrical and thermal conductivity depend strongly on the chemical composition.
The very good electrical conductivity of pure aluminium makes it necessary to use large
currents. Consequently, both thermal stressing and electrode wear are high. The state of
a metal, whether hard or soft, depends on its metallurgical condition. Cold worked
aluminium can generally be better spot-welded than aluminium in a soft condition
(Figure 4500.01.01).
TALAT 4500
3
Surface
Condition
Contact
Resistance
Electrode
Wear
Suitability
for Spot
Welding
Thermal
Conductivity
l
ica n
m sitio
e
Ch p o
m
Co
Electrical
Conductivity
Electrode
Penetration
Resistance
M
e
Co tallu
nd rgi
itio ca
n l
Source: Leuschen
Suitability of Aluminium and its Alloys
for Spot Welding
alu
Training in Aluminium Application Technologies
4500.01.01
Comparison of Physical Properties of Aluminium and Unalloyed Steel
The physical properties of the parts to be welded have a very strong effect on the quality
of the joint. Thus, the electrical conductivity X of pure aluminium at 20 °C is
35 m/Ω mm² and can, depending on the purity and the content of alloying elements, fall
down to 15 m/Ω mm² for other aluminium alloys. In contrast to this, the electrical
conductivity of low alloyed steels is less than 9.3 m/Ω mm². Similarly, the thermal
conductivity of aluminium alloys lies between 100 to 240 W/m K, i.e., about thrice as
large as the corresponding value for steel (Figure 4500.01.02).
Comparision of Physical Properties of Aluminium
and Unalloyed Steel (0.15%C) at RT
Density
Melting
Range
Electrical
Conductivity
Thermal
Conductivity
(g/cm³)
(°C)
(m/Ωmm²)
(W/mK)
Coeff. of
Thermal
Expansion
(10-6 K -1 )
Al 99.5
2.70
646-657
34-36
210-220
23.5
AlMn
AlMg1
AlMgSi0.5
AlMgSi
AlMg0.4Si1.2
AlCuMg1
2.73
2.69
2.70
2.70
2.70
2.80
645-655
630-650
585-650
585-650
590-650
512-650
22-28
23-31
28-34
24-32
26-30
18-28
160-200
160-200
200-240
170-220
160-190
130-200
23.5
23.6
23.4
23.4
23.4
23.0
AlMg3
AlMg5
AlMg4.5Mn
2.70
2.60
2.66
580-650
560-630
574-638
16-24
15-22
16-19
110-170
100-160
110-140
23.0
23.0
24.2
C-Steel
7.85
1510
9.3
50
11.0
Material
Designation
alu
Training in Aluminium Application Technologies
TALAT 4500
Comparision of Physical Properties of Aluminium
and Unalloyed Steel (0.15%C) at RT
4
4500.01.02
Due to the high thermal conductivity of aluminium, a large portion of the heat created is
absorbed by the cooled electrode and the surrounding material. Consequently, a
stationary state of equilibrium between the heat input and output is reached within a
short welding time. For this reason, it is essential that the welding energy for aluminium
be delivered within as short a time as possible. Thus, the current required for welding
aluminium is about twice that for welding steel sheets of the same thickness, and this
although the aluminium alloys have a much lower melting point.
Resistances during Spot Welding of Steel and Aluminium
The formation of the weld nugget depends mainly on the current strength, the resistance
and the welding time. The heat generated can be calculated using Joule's law:
Q = 0.23 ⋅ I2 ⋅ R ⋅ t
where
Q = amount of heat
I = current
R = electrical resistance
t = time
The total resistance is made up of the contact resistances and the material resistances.
The material resistances of the sheets and the contact resistances between the sheets
themselves and between sheet and electrode have very different orders of magnitude for
aluminium and steel. The welding parameters (current, time, force) have thus to be
adapted for the higher electrical and thermal conductivities of aluminium.
Resistance During Spot Welding
of Steel and Aluminium
IS
FEI
R 1 : Material Resistance of Top Electrode
R 2 : Contact Resistance Electrode/ Sheet
R 3 : Material Resistance of Top Sheet
R4 : Material Resistance of Bottom sheet
R 5 : Contact Resistance of Sheet/ Sheet
R 6 : Contact Resistance of Sheet/ Electrode
R 7 : Material Resistance of Bottom Electrode
FEI
Resistance at Start
of Welding
R 1 , R7 .
R 2 , R 4 , R6 .
R 3 , R5 .
Steel
Aluminium
F : Electrode Force
EI
I : Welding Current
S
Negligibly Small
Small
Large
Large
Small
Source: Leuschen
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Training in Aluminium Application Technologies
TALAT 4500
Resistance During Spot Welding
of Steel and Aluminium
5
4500.01.03
An intensive cooling of the electrodes is of paramount importance to avoid any
electrode metal pick-up tendency.
Due to the low resistance of the material aluminium, the spot welding must be carried
out with high welding currents within short welding times using a special electrode
force programme (Figure 4500.01.03).
Constitution of the Oxide Film
The high affinity of aluminium for oxygen, which causes metallic blank aluminium to
be covered at once with a thin, dense and tightly adhering oxide film, has a major effect
on the suitability of aluminium for spot welding. The oxide film has a high thermal
stability and a melting temperature of over 2,000°C and is a non-conductor of
electricity. Diffusion processes govern the formation and growth of this oxide film; its
thickness increases in proportion to the square-root of time.
Constitution of the Oxide Film
P
D
P
M
M
M
5 - 10 nm
S
1 - 2 nm
H
H
Al
Al
D
H
M
S
P
:
:
:
:
:
:
Base Metal
Porous Oxide Film
Heterogeneous Metallographic Constituents
Mixed Oxides
Barrier Layer
Pores
Source: F. Ostermann
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Constitution of the Oxide Film
Training in Aluminium Application Technologies
4500.01.04
The oxide film consists of an almost pore-free barrier layer and a covering porous oxide
layer. The typical thicknesses lie between 0.005 and 0.01 µm (Figure 4500.01.04).
During welding, the high electrode contact pressure causes the sheet surfaces to be
deformed locally. This causes the brittle oxide film to crack, so that the welding current
can now flow through the fine contact bridges; due to the heat developed and the
continuing deformation, the contact bridges are enlarged, causing the contact resistance
to fall rapidly.
TALAT 4500
6
Surface Pretreatment
Chemical surface pretreatment is more suitable than mechanical surface pretreatment for
large aluminium surfaces. It consists of the following steps:
− cleaning
− drying
− etching
− rinsing
− passivation and rinsing
− drying
Most of the etchants used also attack the metallic aluminium once the oxide skin is
removed and, if the contact time is long enough, form a new coating layer which once
again increases the contact resistance. Therefore, the etching parameters must be strictly
adhered to and followed by a thorough rinsing by spraying with cold or warm
(preferably deionised) water (Figure 4500.01.05 and Figure 4500.01.06).
Chemicals
Etching Conditions
Composition
Temperature
Time
1. Sodium
Hydroxide
(NaOH)
Composition
Temperature
Time
2. (Na2CO3) +
Sodium Fluoride
(NaF)
Composition
Temperature
Time
3. Sulphuric Acid
(H2SO4) 95% +
Sodium Fluoride
(NaF)
Composition
Temperature
Time
4. Nitric Acid
(HNO3 ) 66% +
Hydrofluoric Acid
(HF) 40%
alu
Training in Aluminium Application Technologies
TALAT 4500
Composition
Temperature
Time
1.1
50g/l
RT
2min
1.5
200g/l
RT
2min
2.1
75g/l
20g/l
50°C
30sec
1.2
50g/l
60°C
1min
1.6
200g/l
60°C
1min
2.2
75g/l
20g/l
50°C
2min
3.1
100g/l
15g/l
RT
4min
4.1
1050g/l
280g/l
RT
8sec
3.2
100g/l
15g/l
RT
4min
4.2
1350g/l
23g/l
RT
50sec
1.3
50g/l
60°C
2min
1.7
200g/l
60°C
2min
Not Suitable
for G-AlSi and
G-AlSiMg.
Neutralise after
etching in
ca. 30% HNO3
Only for Cu
and Si
contents of 1%
As required,
dip in 50%
HNO3 to
remove deposit
4.3
91g/l
0.5g/l
RT
5.5sec
Surface Pretreatment I
7
Remarks
1.4
200g/l
RT
1min
4500.01.05
Chemicals
Etching Conditions
5. Hydrofluoric
Silic Acid
(H2 SiF) 32.5%
Composition/
Temperature/
Time
6. Sulphuric Acid
(H SO ) 95% +
4
2
Chromium
Trioxide (CrO3 )
Composition/
Temperature/
Time
7. C-Phosphoric Acid
(H3 PO4 ) 85% +
Butanol
(CH3 (CH )OH) +
2
Propanol
Composition/
Temperature/
Time
(CH3 CH(OH)CH3 )
8. O-Phosphoric Acid
(H3 PO4 ) 85% +
Potassium
Dichromate (K
Cr O )
2 2 2
Composition/
Temperature/
Time
9. O-Phosphoric Acid
(H3 PO4 ) 85% +
Nitric Acid (HNO3 )
Composition/
Temperature/
Time
alu
Training in Aluminium Application Technologies
5.1
8.7g/l
RT
3min
6.1
35g/l
148g/l
55°C
20min
7.1
171g/l
324g/l
234g/l
RT
7.5min
8.1
40g/l
0.4g/l
25°C
10min
9.1
1360g/l
49g/l
90°C
2min
Remarks
5.2
38.7g/l
RT
9min
6.2
100g/l
5g/l
60°C
20min
7.2
171g/l
324g/l
234g/l
RT
20min
8.2
40g/l
0.4g/l
25°C
35min
9.2
1360g/l
49g/l
90°C
5min
As Required,
Dip in ca. 30%
HNO to
3
Remove Deposits
Surface Pretreatment II
4500.01.06
Contact Resistance after Surface Pretreatment
A proper surface pretreatment can reduce the contact resistance between electrode and
sheet to values between 3 and 50 µΩ (Figure 4500.01.07). Alkaline solutions, e.g., 10
to 20 % sodium hydroxide solution and mixed acids, are the most commonly used
etchants. The acids added to the solution inhibit the aggressive surface reaction of the
sodium hydroxide solution, making an exact adherence to the optimum etching time less
critical. Ready-for-use etching solutions save the step of preparing such solutions and
can even be applied locally with a brush or sponge.
20
AlMgSi1F32
Contact Resistance R ES
µΩ
AlMgMnF26
15
AlZnMg1F36
10
5
0
1.1 1.2 1.3 1.4 1.5 1.6 1.7 2.1 2.2 3.1 3.2 4.1 4.2 4.3 5.1 5.2
6.1 6.2
7.1 7.2 8.1 8.2 9.1 9.2
Type of Pretreatment
alu
Contact Resistance after Surface Pretreatment
Training in Aluminium Application Technologies
TALAT 4500
8
4500.01.07
Effect of Storage Time on the Contact Resistance
In some cases, the etching step is followed by a post-treatment in 20 to 50 % nitric or
ortho phosphorous acid. This treatment removes any excess etchant residues from the
surface which, if not removed, could lead to harmful deposits, like for example, the
deposition of copper during the etching of copper-containing alloys. Another effect is to
create a uniform thin oxide film (< 0.01 µm). This ensures that the contact resistance
values have low scatter from the very beginning, and that this value changes little even
during long storage times due to the dense surface covering layer (Figure 4500.01.08).
Effect of Storage Time on the Contact Resistance
Contact Resistance R EB
14
7.1 Orthophosphoric Acid Etching AlMgMn F26
4.2 Nitric Acid Post-Treatment
AlMgSi1 F32 }
Etching with Nitric Acid-Hydrofluoric Acid Mixture
AlZnMg1 F36
mW
10
Material
s
FEl
Type of
Pretreatment
AlMgMn F26
2mm
7.5kN
7.1
AlMgSi F32
2mm
7.5kN
4.2
AlZnMg1 F36
2mm
7.5kN
4.2
8
5
4
2
0 0
alu
Training in Aluminium Application Technologies
80
160
240
Storage Time tL
320
480
Effect of Storage Time on the Contact Resistance
h
4500.01.08
Immediately after the last rinsing step, the parts must be dried with warm air, in order to
prevent the formation of flecks which could increase the contact resistance and electrode
metal pick-up tendency. A sure method of preventing formation of flecks is by dipping
the rinsed sheets in methanol before drying them with hot air.
Peltier Effect (Schematic)
During direct current welding, the Peltier effect occurs, causing a larger electrode wear
at the anode than at the cathode. This effect is caused by the heat consumed during
transfer of electricity at the contact cathode-sheet and the heat released at the contact
anode-sheet. This difference in reactions is caused by the corresponding positions of
aluminium and copper in the electrochemical potential series. The additional heat
released at the anode leads to a higher thermal stressing and to lower electrode life
(Figure 4500.01.09).
TALAT 4500
9
Peltier Effect (Schematic)
Element
Voltage (V)
K
-2.925
Na
-2.713
Mg
-2.37
Cu +
Al
-1.66
Zn
-0.763
Fe
-0.440
Cd
-0.402
+Qp
Ni
-0.23
Sn
-0.140
- Qp
Al
Pb
-0.126
H2
0.000
Cu
+0.337
Spot Welding of Al
Ag
+0.799
Direct Current
Cu Pt
+1.20
Au
+1.50
F
+2.87
Electrochemical Potential Series
Based on Hydrogen Electrode at 25°C
Al
Source: Leuschen
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Training in Aluminium Application Technologies
Peltier Effect (Schematic)
4500.01.09
In order to minimise this effect, welding machines have been developed in which the
electrode polarity is changed after each spot weld.
Characteristics of Differently Designed Spot Welding Machines
Besides the commonly used single-phase welding machines for spot welding, one also
now has the three-phase welding machines which can deliver the high currents required.
The latter work either according to the frequency conversion principle or with secondary
rectification. Thus the following requirements can be much better fulfilled:
−
−
reduction of power requirements
improvement of the power factor cos ϕ
− reduction of the unbalanced power loading.
The construction of a single-phase alternating current machine is simple but leads to an
unsymmetrical (unbalanced) power loading at high energies and possesses a low power
factor. Another disadvantage is the limited sheet thickness which can be welded.
Modern high-energy spot-welding machines are constructed nowadays as three-phase
machines with their typical low power requirements and high power factors. Using these
machines, it is possible to weld sheets up to 6 mm in thickness (Figure 4500.01.10).
TALAT 4500
10
Characteristics of Differently Designed
Spot Welding Machines
Three Phase
Frequency
Changer
Single Phase
AC
Three Phase
Rectifier
Pulsed
Current
DC with Rest
Waviness
Two-Phase
with Cyclic
Alternation
Three-Phase
Welding Current
Phase Cut
Sinus Current
(50Hz)
Grid Loading
Two-Phase
Power Requirement
High
Power Factor
0.4-0.7
0.8-0.9
0.9
Maximum Weldable
Al SheetThickness
3+3
6+6
6+6
Low
Low
Characteristics of Differently Designed
Spot Welding Machines
alu
Training in Aluminium Application Technologies
4500.01.10
Relative Voltage Drop of a Resistance Welding Machine
A comparison of the weld throat depths of machines of different constructions, clearly
shows the disadvantage of the single-phase alternating current machine. The voltage
drop here is about 3 to 6 times higher (Figure 4500.01.11).
This fact must be taken into account, especially when large amounts of ferro-magnetic
materials, e.g., clamping and transporting arrangements, reach into the secondary
window.
Relative Voltage Drop of a
Resistance Welding Machine
6
1
Relative Voltage Drop
5
Single-Phase AC
(50Hz)
1
4
3
2
Three Phase Frequency Changer
(30Hz)
3
Three Phase Frequency Changer
(5.6Hz)
2
4
2
3
4
1
Three Phase Rectifier
0
0
200
alu
Training in Aluminium Application Technologies
TALAT 4500
400
600 800
Depth
1000 1200 1400 1600 mm
Relative Voltage Drop of a
Resistance Welding Machine
11
4500.01.11
Factors Influencing the Life of Electrodes
The regulating and controlling possibilities, the welding machine and the parts to be
welded have a strong influence on the life of the electrodes. Reducing the electrode
metal pick-up is of great importance and can be influenced by having a uniform surface
condition obtained through the use of mechanical and chemical pretreatments. This can
be best controlled by measuring the contact resistance between electrode and sheet.
The regulating and controlling possibilities of the welding machine, e.g., chronological
programming of current and force, the resetting behaviour of the electrodes as well as
cooling of the electrode, affect the stroke frequency and consequently the number of
possible weld spots. The higher stroke frequency of roll seam welding necessitates a
direct external cooling of the electrodes (Figure 4500.01.12).
Factors Influencing the Life of Electrodes
Part Controlled Factors
Alloy
Work
Thickness
Surface
condition
Machine
Properties
ac
Welding
Parameters
Cooling
ne
co
n tr
ol le
dF
a ct o
rs
Electrode
Material
Electrode Work
Area
Ma c h
in e
t in g
Se t
Co
s
M
hi
Stroke Time
No. of Spots
Electrode
Life
it
nd
io
n
Source: Leuschen
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Factors Influencing the Life of Electrodes
4500.01.12
Training in Aluminium Application Technologies
Influence of Storage Time on Life of Electrodes
With suitable chemical surface pretreatments, the sheets retain their suitability for
welding over long periods of time, i.e., the attainable life of electrodes also remains
unchanged. In spite of this, efforts must be undertaken to keep the storage time as low as
possible in order to avoid problems caused by dust dirtying the surface of the sheets
(Figure 4500.01.13).
TALAT 4500
12
Influence of Storage Time on the Life of Electrodes
AllMgSi1 F32 Sheets Chemically Pretreated
s = 2mm, CuAg-Electrode, 300mm Spherical Radius
Shear Tensile Force FSZ
kN
12
a) Storage Time t L = 0h
11
12
b) Storage Time t L = 170h
11
0
0
alu
Training in Aluminiu m Application Technologies
100
200
300
Number of Welded Spots
400
500
Influence of Storage Time on the Life of Electrodes
4500.01.13
Influence of Machine Design and Current Type on Life of Electrodes
Since the energy input of direct current machines is more uniform and occurs without
current surges (peaks), higher electrode lives can be obtained than with alternating
currents. The life of the electrodes can be further enhanced by changing the polarity of
the electrodes after each weld spot, thereby reducing the Peltier effect and having a more
uniform wear. Stationary equipment with a higher stiffness also have longer electrode
lives than spot welding tongs (Figure 4500.01.14).
Influence of Machine Design and Current Type on
Life of Electrodes
4.0
kN
Shear Force
3.0
2.5
2.0
1.5
Frequency Changer Machine (Is
Rectifier Machine
(Is
AC Machine
(Is
Tongs
(Is
1.0
0.5
= 32kA)
= 32kA)
= 32kA)
= 32kA)
FEI = 4kN
ts = 4Per
0
0
200
400
600
800
1000
1200
1400
1600
Number of Welded Spots
Source: Leuschen
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Training in Aluminium Application Technologies
Influence of Machine Design and Current Type on
Life of Electrodes
4500.01.14
The distortion in the tong arms gives rise to an added frictional effect in the contact
region electrode-sheet. Similar effects occur when the electrodes are arranged slantingly.
TALAT 4500
13
4500.02
♦
♦
♦
♦
♦
♦
♦
♦
♦
The Process of Resistance Spot Welding
Current-force diagram for spot welding aluminium
Recommended values for spot welding with direct current
Example of a field of suitable welding parameters
Minimum values for spot welding aluminium parts
Effect of weld spot diameter on the shear strength
Correlations between pretreatment, weld strength and electrode life
Repeated tensile stress fatigue strength of one spot samples
Fatigue tests using specimens similar to engineering part
Fatigue strength of spot welded 1.6 mm thick 2024 cl sheets
Current-Force Diagram for Spot Welding Aluminium
A current-force programme in which both parameters change with time, is used to
produce welds of high quality.
An initial force is applied to ensure a good contact between sheet and electrode, thereby
reducing the contact resistance. The force is then lowered, i.e., the contact resistance
between the sheets increases, and the current is allowed to flow.
The rate of current increase (up slope), amplitude, time and current decrease rate (down
slope) can be regulated to suit the alloy to be welded.
Heating of the metals during current flow causes these to soften, so that the force
decreases. Shortly after the maximum current is reached, the force is increased to press
the sheets against each other (forging effect). Similar programmes are mandatory for
welding in the aerospace industry (Figure 4500.02.01).
Guidelines are available for the spot welding of a large number of aluminium alloys.
The parameters stated in the guidelines depend, among others, on the surface condition
and roughness, type of current, material thickness as well as form of electrodes and
material.
The currents used for aluminium are much higher than those required for spot welding
steel, making it necessary to utilise machines with a higher power rating for spot
welding aluminium.
The type of material used for the electrodes has a significant influence on the
operational life. Experience has proved that material strength and hardness are of greater
importance than the electrical conductivity.
TALAT 4500
14
Current-Force Diagram for Spot Welding Aluminium
(2024cl, 1.6 + 1.6mm)
15
I in kA
40
I
F1
F1'
12
F in kN
30
F2
9
20
6
10
3
0
0
0
10
0.2
20
0.4
30
0.6
40
0.8
0
Cycles
sec
50
1.0
Source: SLV Berlin
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Current-Force Diagram for Spot Welding Aluminium
4500.02.01
Training in Aluminium Application Technologies
Recommended Values for Spot Welding with Direct Current
Because of differences in the characteristic physical and mechanical values of steel and
aluminium, a different set of parameters has to be used for the spot welding of these
materials. Because of the high thermal conductivity of aluminium, the welding times
chosen should be as short as possible to prevent the formation of a stationary state of
equilibrium in which the input heat is absorbed by the material and is thus no longer
available for the formation of the weld nugget. Consequently, this means that very high
welding currents have to be used (Figure 4500.02.02). The welding machines take this
fact into account and are designed for the proper power ratings. Since aluminium has a
lower hardness than steel, the electrode forces required for spot welding are lower. High
forces would cause unallowable electrode indentations in the aluminium
Values
Set
Sheet
Thickness
mm
0.35
0.5
0.8
1.0
1.25
1.5
2.0
2.5
3.0
3.5
16
16
16
16
16
20
20
20
25
25
Radius / mm
75
75
75
75
100
100
100
100
100
150
Electrode Force
kN
1.5
1.8
2.2
3.0
3.5
4.0
5.0
6.5
8.0
10.0
Electrode Diameter
mm
Electrode Spherical
Welding Current
kA
Welding Time
18-22 19-24 24-30 25-32 26-34 27-35 30-38 34-42 38-45 44-50
Per
2
2
3
3
4
5
6-8
7-9
8-10
9-12
Weld Nugget
Diameter / mm
3.0
3.5
4.5
5.0
5.5
6.0
7.0
8.0
8.5
9.5
Weld Spot
Diameter / mm
3.5
4.0
5.0
5.5
6.0
6.5
7.7
8.7
9.3
10.3
Source:DVS Pamphlet 2932 T.3
alu
Training in Aluminium Application Technologies
TALAT 4500
Guide Values for Spot Welding with Direct Current
15
4500.02.02
Example of a Field of Suitable Welding Parameters
The welding parameters depend on the most appropriate correlations between current,
time, force and weld strength. These are determined experimentally in test series in
which constant and variable parameters are chosen. The appropriate welding data which
delivers the required standard weld strengths can thus be determined (Figure
4500.02.03).
Example of a Field of Suitable Welding Parameters
2024 cl 1.6 + 1.6 mm
CuCr-Electrode
F1 = 13 kN
F2 = 7 kN
Level of Minimum Average
Tensile Shear DIN 29 878
50
8
35
2
0
d
4
C
40
ur
re
nt
45
6
55
kA
W
el
Electrode Force
10
30
3
Source: Krüger
alu
Training in Aluminium Application Technologies
4
6
8 Cycles 10
Weld Time
Example of a Field of Suitable Welding Parameters 4500.02.03
Minimum Values for Spot Welding Aluminium Parts
Analogous to the parameter settings, geometrical values like weld spot and nugget
diameter as well as distance from edge depend on the material. The amount of overlap
and the distance between the weld spots depend on the electrical conductivity of the
work material, i.e., the distance chosen between the weld spots increases with increasing
electrical conductivity.
The diameters of weld spot and weld nugget depend on the electrode geometry and on
the number of weld spots. Therefore, the effect of increasing electrode diameter on the
weld spot strength must be regularly controlled. The minimum strengths are laid out in
the appropriate regulations (Figure 4500.02.04).
TALAT 4500
16
Minimum Values for Spot Welding Aluminium Parts
Material
AlMg5Mn
Sheet Thickness t
Min. Weld Nugget
Diameter dL
Min. Weld Spot
Diameter dp
Min. Distance from edge v
Electrode
b
Diameter dE
Min. Flange
Width
t
Electrode
b
Diameter dE
Min. Overlap
All Values in mm
AlMg0.4Si1.2
1.0 1.15 1.25 1.5 2.0 2.5 3.0 1.0 1.15 1.25 1.5 2.0 2.5 3.0
4.0 4.3 4.5 4.9 5.7 6.3 6.9 4.5 4.8 5.0 5.5 6.3 7.1 7.8
4.4 4.7 5.0 5.4 6.3 7.0 7.6 5.0 5.3 5.5 6.0 7.0 7.8 8.6
5.5 5.9 6.3 6.8 7.9 8.8 9,5 6.3 6.6 6.9 7.5 8.8 9.8 10.8
16
20
16
20
17.5 18.0 19.0 19.5 23.5 25.0 26.5 18.5 19.0 19.5 20.0 24.5 26.0 27.5
16
20
16
20
13.0 14.0 14.5 15.5 18.0 19.5 21.0 14.5 15.5 16.0 17.0 19.5 21.5 23.5
dL≥ 4.0*√t
dL≥ 4.0*√t
b ≥ dE / 2 + r + v + x
v ≥ 1.25∗dp
dp = 1.1*dL
r = 1.5*t
ü = 2*v+y
x = Flange Width Tolerance + Flange Misalignment + Electrode Misalignment = 2.5
y = Sheet Misalignment + Electrode Misalignment = 2.0
Source: Leuschen
alu
Training in Aluminium Application Technologies
Minimum Values for Spot Welding Aluminium Parts
4500.02.04
Effect of Weld Spot Diameter on the Shear Strength
The load-carrying capacity of the joint depends on the weld spot diameter. Depending
on the welded materials, weld spot diameter and shear strength increase proportionally
(see Figure 4500.02.05). Heat-treatable alloys which have been age hardened, lose this
hardening effect in the heat affected zone of the weld. A heat treatment of such
components is, in principle, possible but its practical use is strongly limited by the part
size and possible distortion expected. Therefore, the unavoidable loss in strength is
compensated for by increasing the weld spot diameter. Larger weld spot diameters are
necessary to attain the required shear strength with softer materials.
TALAT 4500
17
AlMg5Mn
kN
8
Tensile Shear Force
AlMg0.4Si1.2
t = 2.0 mm
t = 1.5 mm
t = 1.25mm
7
6
5
4
3
2
0
0
4
5
6
7
8
Weld Spot Diameter
9
10 mm
Source: Leuschen
alu
Training in Aluminium Application Technologies
Effect of Weld Spot Diameter on Tensile Shear Strength
4500.02.05
Correlations between Surface Pretreatment, Weld Strength and Electrode Life
The mechanical pretreatment can be carried out with simple equipment and does not
require any special rooms. The softer alloys (pure aluminium, AlMn) tend to "smear"
easily so that proper care must be taken while working with them. The same care must
be taken for plated alloys, in order to prevent a mechanical removal of the thin plating.
Small areas can be mechanically pretreated with hand-held or rotating (ø > 200 mm)
brushes made of stainless steel or also with steel wool. Grinding is carried out using
grinding wheels or grinding bands. Abrasive powder with uniform particle size is used
to obtain a fine surface finish. Overheating is prevented by using moderate grinding
pressures, appropriate operating speeds (30 to 40 m/min) as well as by introducing
additives (fat, paraffin) to the abrasive powder.
A disadvantage of mechanical surface pretreatments is the roughening of the surface.
The contact resistance can only be reduced down to 50 to 100 µΩ, the associated
heating at the interface electrode/sheet surface leads to electrode metal pick-up. The
mechanical pretreatment produces an active surface which oxidises rapidly to form a
new oxide film. Consequently, the contact resistance rises sharply within short storage
times, increases of 20 µΩ occurring within about 20 min (Figure 4500.02.06).
TALAT 4500
18
Alloy 6082-T6; t = 2mm ; Fel = 8kN ; Rel = 300mm
Shear Tensile Force in kN
12
Etched
Is = 49kA
10
5
0
100
200
300
12
400 500 600
No. of Points n
700
800
900
1000 1100
Brushed
Is = 52.2kA
10
8
6
0
100
200
300
400 500 600
No. of Points n
700
800
900
1000 1100
Correlations between Surface Pretreatment,
Weld Strength and Electrode Life
alu
Training in Aluminium Ap plication Technologies
4500.02.06
Repeated Tensile Stress Fatigue Strength of One-Spot Samples
The fatigue behaviour of materials can be ascertained at values of 2 x 107 cycles and
more. Weld defects like pores and cavities which are not located at the weld nugget
edges have hardly any influence. Cracks and reductions in cross-section due to too high
electrode pressures, reduce the fatigue strength sharply. The amount of overlap also has
a significant effect on the fatigue behaviour (Figure 4500.02.07).
Tensile Force Amplitude
2.0
kN
ü
1.5
ü = 15mm
ü = 40mm
t = 2.0 mm
t = 1.25mm
1.0
0.5
0
104 2x104 5x104
105 2x105 5x105 106 2x106 5x106
No. of Cycles
107 2x107 5x107
108
Source: Leuschen
alu
Fatigue Strength of Single-Spot Welded Samples
4500.02.07
Training in Aluminium Application Technologies
Fatigue Tests Using Specimens Similar to Engineering Parts
The results of fatigue tests can only be compared using standardised specimen forms.
The load transfer and additional bending are decisive factors for single and double
layered joints. The specimens have to be prepared singly, i.e., they may not be taken out
of the sheet or the constructional part. The welding is carried out using special
arrangements, and the welding procedure must always be recorded (Figure 4500.02.08).
TALAT 4500
19
Test Specimen A
( Part 1 and Part 5 )
Load Transfer = 0
Additional Bending = 0
Part 1 Part 5
Metallurgical Influence of a Single
Weld Spot on the Base Material
Properties ( not bridge-connected ).
Peltier Effect must be considered while
Welding with DC.
Test Specimen D
( Part 3 and Part 4 )
Load Transfer = Medium
Additional Bending = High
Example: End of a Skin
Stiffener
Electrode Diameter to be considered
while Welding, eventually both surfaces
to be treated; compare Inside Dimensions
of U-Profile
Test Specimen E
( Part 3 Once, Part 4 Double )
Load Transfer = High
Additional Bending = 0
Part 4
Part 3
2
1
3
1
Part 4
Test Specimen B
( Part 1 and Part 10 )
Load Transfer = Low
Part 1
Part 3
2
1
3
Part 10
Part 8
4
2
1
3
5
Example: Base Structure
Test Specimen C
( Part 2 Double, displaced
relatively by 180o )
Load Transfer = low
Additional Bending = Low
Part 2
2
1
Test Specimen F
( Part 8 Double )
Load Transfer = High
Additional Bending = High
Test Specimen G
( Part 6 Double, Parts 7
and 9 Single )
Load Transfer = High
Additional Bending = High
Example: Cutting Location
3
5
4
6
2
1
3
8
7
9
Part 9
Part 7
Part 6
2
8
11
5
1
7
10
4
3
9
12
6
Part 6
Source : DVS Pamphlet No. 2709
4500.02.08
Component-like Specimens for Fatigue Tests
alu
Training in Aluminium Application Technologies
Fatigue Strength of Spot Welded 1.6 mm Thick 2024 cl Sheets
Experiments have shown that the effect of specimen form on the fatigue strength is
much greater than that of other variables like lower or higher current, cracks, corrosion
etc. The effect of the added bending due to specimen form is clearly visible. Therefore, a
careful evaluation of the results is essential (Figure 4500.02.09).
200
R = 0.1
10%
5*104
Pü = 50%
90%
No. of Cycles
Low
High
Cracks, Sprays
Low
High
Low
High
Corrosion
Surface Prot.+Corr.
B
Low
High
"Curved"
Corrosion
Surface Prot. + Corrosion
A
Lower Reference Value
Low
High
Corrosion
Surface Protection + Corrosion
20
10
9
8
7
6
5
Low Static Shear Strength
High
High, without Plate
30
Low
High
Cracks, Sprays
Corrosion
Surface Protection + Corrosion
2*106
BASE MATERIAL
Stress Amplitude [N/ mm² ]
100
90
80
70
60
50
40
D
E
F
G
C
Specimen Types According to DVS 2709
Source: Krüger, SLV Berlin
alu
Training in Aluminium Application Technologies
TALAT 4500
Fatigue Strength of Spot Welded 1.6mm Thick
2024cl Sheets
20
4500.02.09
4500.03 Quality Assurance
•
Allowable imperfections of spot welds in aluminium alloys (according to
aeronautical standards) determined by metallography
• Allowable imperfections of spot welds in aluminium alloys (according to
aeronautical standards) determined by x-ray examination.
Allowable Imperfections of Spot Welds in Aluminium Alloys (According to
Aeronautical Standards) Determined by Metallography
The geometrical characteristics and allowable defect dimensions for spot welds depend
on the corresponding evaluation group (EG). The EG I has the strictest requirements
(crack and inclusions are not allowed in this evaluation group) (Figure 4500.03.01).
Allowable
Imperfections
of Spot Welds in
Aluminium Alloys
Determined by
Metallography
Ser.
No.
Characteristics
I
1
t2
2
Inclusions, cavities
3
Length of cavity or inclusions
4
Distance of cavity or inclusion
from edge
5
Penetration of cavity or
inclusion
6
Penetration depth of nugget
within 0.8 dr
7
Nugget diameter d ²)
8
Gap thickness 0,5 dr
9
Indentation
depth or
bulging
t1
Plating
II
Cracks
III
Inclusion or Cavity
Bulging
Distance
Inclusion or Cavity
Indentation
Penetration Depth
Gap
Width
Weld Nugget
Penetration
EG
Inclusion or Cavity
Length
0,8 dr
dr 1)
0,5 dr
Aerodynamic
Not aerodynamic
I
II
III
II
III
II, III
II
III
I, II, III
I, II, III
I, II
III
I, II, III
I
I
II,III
I, II, III
I, II
III
Required or allowable value
Not allowed
Longest single crack 10 % of weld nugget
penetration, measured from nugget middle
Longest single crack 30 % of weld nugget
penetration, measured from nugget middle
Not allowed
2 (in number)
3 (in number)
≤ 0.15 dr 1)
≤ 0.25 dr
≥ 0.15 dr
≤ 0,25 dr
≤ 0.50 dr
≥ 0.2 t1 and ≤ 0.9 t1
≥ 0.2 t1 and ≤ 1.0 t1
≥ 0.2 t1 and ≤ 0.8 t1
≥ 0.2 t1 and ≤ 0.9 t1
see table
≤ 0.05 (t1 + t2) or ≤ 0.15 mm larger of
≤ 0.05 (t1 + t2) or ≤ 0.12 mm the values
≤ 0.075 (t1 + t2) or ≤ 0.15 mm allowed
≤ 0.2 t1, bu ≤ 0.1 mm
For t1 ≤ 0.2 mm, allowed 0.3 t1
For t1 > 0.2 mm, allowed 0.1 t1 or 0.13 mm
For t1 ≤ 0.2 mm, allowed 0.2 t1 or 0.2 mm
For t1 > 0.2 mm, allowed 0.4 t1
Overlapped spot with roller
I, II, III
see table
Penetration Depth 10
seam weld
on the 0,8 dr line
1
) dr = the weld nugget diameter measured on the metallographic specimen
²) dr = the required minimum weld nugget diameter
Source: DIN 29 878
alu
Training in Aluminium Application Technologies
Allowable Imperfections of Spot Welds in
Aluminium Alloys Determined by Metallography
4500.03.01
Characteristics which can be made visible in a metallographic section allow a much
more comprehensive evaluation of the quality of the spot weld joint than radiographs.
The effort of producing the former is also much higher.
Allowable Imperfections of Spot Welds in Aluminium Alloys (According to
Aeronautical Standards) Determined by X-Ray Examination
Inclusions and cavities can also be determined using the radiographic testing method.
An important factor here is the location distance of the defect from the edge. Defects
TALAT 4500
21
located within the interior of the weld spot have a lower notch effect and are therefore
not as dangerous as those located at the spot boundaries (Figure 4500.03.02).
Allowable Imperfections of Spot Welds
in Aluminium Alloys Determined by
X-Ray Examination
Inclusion or Cavity
Distance from Edge
Inclusion or Cavity
Length
dr
Ser.
No.
Characteristics
1
Cracks
2
Inclusions,
cavities
3
Inclusion or
cavity length
4
Total area of
inclusions or cavities
5
Inclusion or cavity
distance from edge
EG
Allowable Values
I
II
III
I
II
III
II
III
II, III
II, III
I, II
not allowed
10 %
30 %
not allowed
2 (in number)
4 (in number)
≤ 0,15 dr
≤ 0,25 dr
< 10 % of nugget area
< 5 % of nugget area
≤ 0,15 dr
Source: DIN 29 878
alu
Training in Aluminium Application Technologies
Allowable Imperfections of Spot Welds in
Aluminium Alloys Determined by X-Ray
Examination
4500.03.02
4500.04 Literature
B. Leuschen, Joining aluminium body materials, in F. Ostermann (ed.) „Aluminium
Materials Technology for Automobile Construction“,MEP Mechanical Engineering
Publications Ltd., London, 1993
TALAT 4500
22
4500.05 List of Figures
Figure No.
Figure Title (Overhead)
4500.01.01
4500.01.02
Suitability of Aluminium and its Alloys for Spot Welding
Comparison of Physical Properties of Aluminium and Unalloyed Steel
(0.15% C) at RT
Resistances During Spot Welding of Steel and Aluminium
Constitution of the Oxide Film
Surface Pretreatment I
Surface Pretreatment II
Contact Resistance after Surface Pretreatment
Effect of Storage Time on the Contact Resistance
Peltier Effect (Schematic)
Characteristics of Differently Designed Spot Welding Machines
Relative Voltage Drop of a Resistance Welding Machine
Factors Influencing the Life of Electrodes
Influence of Storage Time on Life of Electrodes
Influence of Machine Design and Current Type on Life of Electrodes
4500.01.03
4500.01.04
4500.01.05
4500.01.06
4500.01.07
4500.01.08
4500.01.09
4500.01.10
4500.01.11
4500.01.12
4500.01.13
4500.01.14
4500.02.01
4500.02.02
4500.02.03
4500.02.04
4500.02.05
4500.02.06
4500.02.07
4500.02.08
4500.02.09
4500.03.01
4500.03.02
TALAT 4500
Current-Force Diagram for Spot Welding Aluminium
Guide Values for Spot Welding with Direct Current
Example of a Field of Suitable Welding Parameters
Minimum Values for Spot Welding Aluminium Parts
Effect of Weld Spot Diameter on Tensile Shear Strength
Correlations Between Surface Pretreatment, Weld Strength and Electrode
Life
Fatigue Strength of Single-Spot Welded Samples
Component-like Specimens for Fatigue Tests
Fatigue Strength of Spot Welded 1.6 mm Thick 2024 cl Sheets
Allowable Imperfections of Spot Welds in Aluminium Alloys Determined
by Metallography
Allowable Imperfections of Spot Welds in Aluminium Alloys Determined
by X-Ray Examination
23